It could be possible that Pip could have experienced some brain damage from the malnutrition, which would have given him some neurological problems. He could've just not had very much mental simulation as a chick, which will result in some low-functioning adult birds. You can try giving him little challenges, like simple foraging toys (for foraging food), which can give him some mental simulation, which will improve his mental ability. "Foraging toys" can be as simple as a small Tuboware filled with shredded paper and treats, where they require to search for the treats hidden.
If you have trouble giving him nutri berries, you can try shredding them, to where they will look more like seeds. One would think him seeing coco eat it would let him know that it's food, but perhaps that's just not always the case.
It he does have neurological problems, I can tell you, somewhat from personal experience (being diagnosed with autism and all), it's not likely that you can 100% correct the problems, but you can get pretty darn close.

Even some of the most basic training provides the exercise of mind, and you can't get any more basic than target training.
Does he know to step up, yet? If he does, then you can already teach him to wave on command. If not, then you can start with the most basic of them all: target training, where you get the bird to "bite" (or simply touch the stick with their beak) the end of a stick for a treat. This does give mental simulation, because he'll learn to make the connection that "touching stick with beak = treat". Target training is also good, because through it, you can teach the bird to do much more. I can give you some guides and articles, if you're interested.

Speaking of which, I need to get Lucy back into her training.

we've been slacking off for a while, now.
As for Coco's aggression towards Pip, it sounds like Coco doesn't accept pip as a member of the flock. You can try fixing that, once you have some millet sprays (if your uncle will get it for you at the October bird expo). You can put a couple sprays on top the cage, as well as putting them on top of the cage. They can eat from their own sprays while near each other. The more comfortable they get, the shorter you can make their distances. The goal will be for them to eat from the same spray, without any problems.
Now when you say Pip looks "a bit off", I'm curious about what you mean. Does he have some behaviors that seem peculiar?